Prior to the present invention, it is generally recognized in the art that fluid pressure responsive railway car brake actuators have been used on passenger transit type vehicles.
These brake actuators generally consist of a tubular housing member which is equipped with a cylinder portion adjacent one end thereof. A piston member is slidably disposed within such cylinder portion of the brake actuator. Such piston member is positioned to slide under the action of a predetermined fluid being communicated to the cylinder portion under pressure and against the action of at least one elastic type spring retraction means. Such piston member includes a generally hollow shaft member connected thereto, which normally does not rotate in relation to the tubular housing and in which is axially connected a threaded rod member to operate the braking means disposed on the vehicle.
In this particular prior art type brake actuator, the threaded rod member is likewise a nonrotating member with respect to such tubular housing member. During the braking action of the threaded rod member, a clutch-type, self-adjusting mechanism is provided which will cause the relative axial positions between the threaded rod member and the hollow shaft member to be changed as necessary when such above-mentioned vehicle braking means begins to exhibit excessive brake shoe wear. Such self-adjusting mechanism generally consists of an internally threaded stopping ring member which is threadedly engaged with a threaded rod member and includes a front coupling portion which matingly engages with a corresponding front coupling portion of the hollow shaft member to block the rotation of the stopping ring member, in relation to the generally hollow shaft member, and elastic spring retraction means are provided to restore the engagement of these front coupling portions, by the screwing of the retaining element in relation to the threaded rod member, after an axial separation between these front coupling elements has been achieved.
In addition, there is a friction ring member disposed between the tubular housing member and the threaded rod member. Such friction ring member acts on the threaded rod member and slides together with such threaded rod member in relation to the tubular housing member, during the normal braking operation of such fluid pressure responsive brake actuator, for a predetermined brake stroke length.
There are limit stop means positioned on the tubular housing member in order to limit the longitudinal movement of such friction ring member at the completion of such predetermined stroke length and to additionally allow for an axial sliding movement of the threaded rod member in relationship to the hollow shaft member, in order to automatically adjust the stroke length of the brake actuator for any excess wear exhibited by the vehicle braking means.
As is evident, therefore, in this known prior art type fluid pressure responsive brake actuator the threaded rod member will slide out of the hollow shaft member every time the advancement of such hollow shaft member is greater than the predetermined normal brake stroke length as defined by the above-mentioned limit stop means.
This brake stroke overtravel length may be due, in most cases, to the normal wearing of the brake shoes disposed on the vehicle, the braking actuator itself and, also, it can be at least partially due to the normal elastic give of the brake force transmission elements, which connect the brake actuator to the vehicle brake unit.
These brake force transmission elements generally consist of two caliper arm members. One of such caliper arm members being connected, in a known manner, to the brake actuator housing member and the other one of such caliper arm members being connected to the free end of such threaded rod member. These caliper arm members include a pair of friction brake pads which are adapted to engage a disc of the railway car or similar passenger transit type vehicle.
The required action to adjust for the above-mentioned play and/or elastic give of the brake actuator has generally been found to be negligible in nature. In other words, such play and elastic give encountered is normally not of sufficient magnitude to have any adverse effect on the correct operation of the fluid pressure responsive brake actuator. However, in some particular known applications, the adjustment action required to be taken by such self-adjusting mechanism to take up the play encountered due to such elastic give of the brake actuator is undesirable and must, therefore, be eliminated.
For a more complete understanding of known prior art type fluid pressure responsive brake actuators reference can be made to U.S. Pat. Nos. 4,676,346 and 4,895,228, the disclosures of which are incorporated herein by reference thereto.